1. Field of the Invention
The present invention relates to a novel organic compound for a light-emitting device, a light-emitting device which is used for a plane light source or a flat panel display, and to an image display apparatus.
2. Description of the Related Art
In an old example of an organic light-emitting device, a voltage has been applied to an anthracene evaporated film to emit light (see, Thin Solid Films, 94 (1982), 171). In recent years, applied research has been vigorously conducted on a transformation of an organic light-emitting device into a light-emitting device having high-speed response and high efficiency, including developments of materials for the device. This is because the organic light-emitting device has such advantages that its area can be increased more easily than an inorganic light-emitting device, desired colors can be developed through the developments of various new materials, and it can be operated at a low voltage.
For example, as discussed in Macromol. Symp. 125, 1 to 48 (1997), an organic EL device is generally composed of a transparent substrate, a pair of upper and lower electrodes formed on the transparent substrate, and an organic layer interposed between therebetween including a light-emitting layer.
Recently, investigation has been made into a device utilizing not only conventional light emission utilizing fluorescence ascribable to a transition from an excited singlet state to a ground state but also phosphorescence via a triplet exciton as typified by technologies described in each of “Improved energy transfer in electrophosphoresent device”, D. F. O'Brien et al., Applied Physics Letters, Vol 74, No 3, p 422 (1999) and “Very high-efficiency green organic light-emitting devices based on electrophosphorescence”, M. A. Baldo et al., Applied Physics Letters, Vol 75, No 1, p 4 (1999). In each of those documents, an organic layer having a four-layer structure has been mainly used. The organic layer includes, from an anode side, a hole-transporting layer, a light-emitting layer, an exciton diffusion-prevention layer, and an electron-transporting layer. Materials used are a carrier-transporting material and a phosphorescent material Ir(ppy)3 shown below.

A variety of light ranging from ultraviolet light to infrared light can be emitted by changing the types of fluorescent organic compounds. In recent years, active research has been conducted on various compounds.
In addition to an organic light-emitting device using any one of the low-molecular-weight materials as described above, an organic light-emitting device using a conjugated polymer has been reported by the group at the University of Cambridge (see, Nature, 347, 539 (1990)). In this report, there has been observed light emission from a single layer by forming polyphenylenevinylene (PPV) into a film by means of a coating system.
As described above, an organic light-emitting device has recently showed significant progress. The organic light-emitting device can be transformed into a high-speed response, thin, and lightweight light-emitting device which can be driven at a low applied voltage and has high luminance and a variety of emission wavelengths, thus suggesting possibilities of a wide variety of applications.
However, under the present conditions, a light output higher in luminance and higher conversion efficiency are being sought. In addition, there still remain a large number of problems in terms of durability such as changes over time during long-term use and deterioration due to an atmospheric gases containing oxygen or moisture.